Lab Report: Fuel Management

Strategies to improve your glycogen storage

When you run, your body burns a mixture of carbohydrate and fat. Your body stores carbohydrate as glycogen in your muscles and liver (the fitter you are the more you store), which is broken down to glucose as needed. The harder you run, the more carbohydrate you use. As your glycogen stores become progressively more depleted during a run, your body conserves what’s left by relying more on fat. Because fat is about 15 percent less efficient than carbohydrate as an energy source, when you run low on glycogen you slow down. You can therefore improve your training and race performance by managing your glycogen stores.

Glycogen loading before long races

Glycogen depletion is a key limiting factor in races lasting longer than about 90 minutes. Studies in the 1960s showed that athletes can substantially increase their muscle glycogen stores by doing a long workout seven days before a competition, then eating a low-carbohydrate diet for three days, followed by a high-carbohydrate diet (70-80 percent of calories from carbohydrate) for the three days preceding the race. The long run depletes the body’s glycogen stores and the three days of low carbohydrate intake keeps them low, which signals the body to store as much glycogen as possible. The downside of the carbohydrate depletion phase is irritability, weakness (which is not great psychologically before a key race) and immune system suppression.

Later studies showed that you can increase your glycogen stores to similar levels without the depletion run and low-carbohydrate phase by tapering training and eating a high-carbohydrate diet during the last three days before a race. To ensure glycogen stores are completely topped up for a marathon, I recommend a modified approach with a one-day depletion phase. For a marathon on Sunday, you would do a moderate depletion run (e.g., one hour with 20 minutes at marathon race pace) on Wednesday morning and eat low carbs throughout the day, then after a light run on Thursday morning start carbohydrate loading. The one-day depletion provides a strong stimulus to your body to store glycogen while minimizing the side effects.

You should expect to gain a couple of pounds when you carbo-load because your body stores 2.6 grams of water for every gram of glycogen. The added weight is unavoidable, and the stored water may help prevent dehydration during the race.

Training low and racing high

Renowned exercise physiologist Bengt Saltin, Ph.D., and colleagues at the University of Copenhagen have suggested that allowing muscle glycogen levels to become depleted may lead to improvements in athletes’ adaptation to training. Studies have found that exercising with low muscle glycogen levels leads to greater activation of genes involved with several types of beneficial training adaptations. One specific adaptation that is enhanced by glycogen depletion is the ability of the body to store glycogen. Depletion of muscle glycogen leads to increased activity of glycogen synthase, which is the enzyme that increases glycogen storage. So, rather than consistently topping up your stores and using a carbohydrate drink during training, there may be a benefit to allowing your glycogen tank to run low during some workouts.